A measurement of electromagnetic (EM) properties of earth formation penetrated by a borehole has been used for decades in hydrocarbon exploration and production operations. The resistivity of hydrocarbon is greater than saline water. A measure of formation resistivity can, therefore, be used to delineate hydrocarbon bearing formations from saline water bearing formations. Electromagnetic borehole measurements are also used to determine a wide range of geophysical parameters of interest including the location of bed boundaries, the dip of formations intersecting the borehole, and anisotropy of material intersected by the borehole. Electromagnetic measurements are also used to “steer” the drilling of the borehole.
Borehole instruments, or borehole “tools”, used to obtain EM measurements typically comprise one or more antennas or transmitting coils which are energized by an alternating electrical current. Resulting EM energy interacts with the surrounding formation and borehole environs by propagation or by induction of currents within the borehole environs. One or more receivers respond to this EM energy or current. A single coil or antenna can serve as both a transmitter and a receiver. Parameters of interest, such as those listed above, are determined from the response of the one or more receivers. Response of one or more receivers within the borehole apparatus may be telemetered to the surface of the earth via conveyance means that include a wireline or a drill string equipped with a borehole telemetry system; such as mud pulse, sonic or electromagnetic telemetry. Alternately, the response of one or more receivers can be stored within the borehole tool for subsequent retrieval at the surface of the earth.
Standard induction and wave propagation EM tools are configured with transmitter and receiver coils with their magnetic moments aligned with the major axis of the tool. More recently, induction tools with three axis coils and wave propagation MWD or LWD tools with antennas (coils) whose magnetic moments are not aligned with the tool axis are being produced and used. These MWD or LWD propagation tools, with antenna dipole axes tilted with respect to the tool axis, can locate boundaries with resistivity differences as a function of tool azimuth. Tools with coils aligned with the tool axis cannot locate boundaries with resistivity changes as a function of tool azimuthal angle. The azimuthal resistivity response feature of an electromagnetic MWD or LWD tool is most useful in direction or “geosteering” the drilling direction of a well in a formation of interest. More specifically, the distance and direction from the tool to a bed (such as shale) bounding the formation of interest, or water interfaces within the formation of interest, can be determined from the azimuthal resistivity response of the tool. Using this information, the drill bit can be directed or “steered”, in real time, to stay within the formation zone of interest so as to avoid penetrating non hydrocarbon bearing formations with the borehole.
Prior art MWD or LWD tools that make azimuthal EM measurements employ a combination of separate axially aligned antennas and antennas whose magnetic moments are tilted at an angle with respect to the tool axis. Such tools, for example, are described in U.S. Pat. No. 6,476,609 issued to Bittar, and U.S. Pat. No. 6,297,639 issued to Clark et al. These tools have a fixed inclination and azimuth response, and can only transmit or receive magnetic fields at a particular orientation relative to the tool. These patents include a rotational position sensor and a processor to identify the azimuthal angle of the magnetic moments as the tool rotates during drilling. Furthermore, the antennas with different dipole orientations located at different axial spacings along the length of the tool lack a common dipole origin point. This fact precludes vector addition of the dipole moments to form a new dipole moment, in any direction, with the same origin point. Multiple antennas at differing axial spacings also increase tool production and maintenance cost, and further reduces mechanical tool strength.
Electromagnetic antennas have been designed for MWD or LWD tools for the past three decades. The use of highly magnetic permeable material in the design of these antennas has been around for the past two decades and antennas which generate a magnetic field in directions other than the tool axis directions have been designed mostly in the past decade. U.S. Pat. No. 4,536,713 issued to Davis et al. describes a high permeability magnetic material disposed in a drill collar used for measuring mud resistivity outside the collar in the annulus region between the drill collar and the borehole wall. U.S. Pat. No. 5,138,263 issued to Towle describes placing magnetic material between an antenna wire and an MWD collar to electromagnetically couple the antenna signal to the formation.
U.S. Pat. No. 6,181,138 issued to Hagiwara describes an arrangement of three antennas disposed around a drill collar in which each antenna is composed of a coil wire disposed within a plane and oriented at an angle with respect to the tool axis. Each of the three antennas is basically a wire around the outside of a usually steel drill collar, wherein the path of the wire is located in a plane intersecting the drill collar. The normal vector to this plane can be described as having an inclination angle and an azimuthal angle. Azimuthal angle as it is being used here is the angle around the tool perpendicular to the tool axis. The origin of the vector is the center of the plane containing the antenna. All of the three antennas have the same centroid or geometric center and, as such, produce magnetic vectors which have a common origin or are co-located. The patent also describes on the same tool additional antennas spaced apart along the tool axis and oriented at a second angle with respect to the tool axis. The additional antennas are disposed within a plane which makes an angle of zero degrees in the same manner that standard wave propagation resistivity tools are constructed. The patent also discloses using the antennas in combination with a rotational position sensor and a processor contained within the MWD tool. The patent also describes combining the three antennas to electrically orient the antenna magnetic dipole moment to any azimuthal angle, but cannot change the inclination angle. This antenna design places coils around a drilling collar in a region of reduced diameter or “necked down” region. It is well known in the art that reducing the outer diameter of a drilling collar weakens it in that area and causes the collar to be more prone to mechanical failure. In this design also the coils must be covered with a non-conducting layer which must go all the way around the collar for the extent of the tilted coils. Non-conductive coverings presently used in the art such as fiberglass, rubber, epoxy, ceramics or plastic are subject to wear due to abrasion which occurs between the tool and the borehole wall, and are not as strong as the collar material. Because the non-conducting region must encircle the collar it is likely to contact the borehole wall unless the collar is further “necked down” causing further weakness. An extreme penalty is paid by “necking down” drilling tubulars. It is well known to those skilled in the art that reducing the outer diameter of a cylindrical member reduces the torsional and bending stiffness proportional to the forth power of the radius. For example, reducing the diameter of a 5 inch (12.7 centimeter) tubular to 4 inches (10.2 centimeters) reduces the torsional and bending stiffness by 59%.
U.S. Pat. No. 6,476,609 issued to Bittar describes at least one antenna disposed in a plane and oriented at an angle with respect to the tool axis and another antenna displaced along the tool axis from the first antenna and disposed in a plane and oriented in a different angle with respect to the tool axis. This patent also includes a rotational position sensor and a processor.
U.S. Pat. No. 7,038,457 issued to Chen and Barber, and U.S. Pat. No. 3,808,520 issued to Runge, describe co-located triaxial antenna construction in which three orthogonal coils are wound around a common point on a borehole logging tool. These patents describe the virtues of having antennas with three orthogonal dipole moments all passing through the same point in the center of the logging tool. The teachings of both patents are more suitable for tools conveyed into a borehole by wireline, rather than tools used in drilling a borehole, because the disclosed coil windings would compromise the strength and durability of an MWD or LWD tool. Runge describes a triaxial antenna located in the center of a tool with non-conducting tool housing or “mandrel” around it. This design is clearly not appropriate for MWD or LWD embodiment. It is known to those of ordinary skill in the MWD or LWD art that a non-conducting tool body does not have the strength to support the severe mechanical requirements of tools used in drilling. Chen and Barber describe a technique for implementing an antenna structure with co-located magnetic dipole moments in which the transverse coils penetrate a mandrel through openings in the tool body. While this may be appropriate for wireline applications, openings in the tool body in which a coil is placed will cause weakness in the tool body. In addition provision must be made for drilling fluid or drilling “mud” to flow down within the body of an MWD or LWD tool. This mud usually flows in a conduit or channel in the center of the MWD or LWD tool, which is typically a drill collar. Embodied in a MWD or LWD system, the Chen and Barber design must somehow be modified to divert the mud away from the coils and the openings in the tool body thereby adding complexity and cost to the manufacture of the tool. Another problem encountered in embodying the Chen and Barber design as an MWD or LWD system is that, owing to the required non-conductive covering which is disposed around the circumference of the tool, the coils are not protected from abrasion which occurs between the tool and the borehole wall during drilling.
A more robust antenna design suitable for MWD or LWD application is described in U.S. Pat. No. 5,530,358 issued to Wisler et al. This antenna is integrated into a drilling tubular affording maximum strength and abrasion resistance, One of the key components of the Wisler et al. system is the antenna is composed of grooves and wire pathways disposed beneath the surface of the drilling tubular surface to avoid any abrasion and so as not to reduce the strength of the tubular. The patent further discloses disposing magnetic material between the wire and the grooves.
U.S. Pat. No. 7,057,392 issued to Wang et al describes an antenna with grooves on the outside of the tool that are oriented “substantially orthogonal to the tool axis”. The antenna construction and grooves are similar to those described in U.S. Pat. No. 5,530,358.